1. Field of the Invention
The present invention relates to a method for determining the blood recirculation in a vascular access.
The present invention applies in particular to determination of the blood recirculation in a vascular access during a dialysis treatment, which will be taken as a non-limiting example in the following description.
2. Description of Related Art
It is known that blood is composed of a liquid part, blood plasma, and of a corpuscular part formed by cells, including the red corpuscles which contain haemoglobin. In cases of renal insufficieny, undesirable substances of low moleular weight are also found in the blood, and these can be removed by a dialytic treatment carried out using a dialysis apparatus.
Dialysis treatments have an efficiency which is defined by the ratio between the volume of blood purified during the dialysis session, and the patient""s total blood volume.
FIG. 1 illustrates a simplified model of the blood flows in a patient/extracorporeal circuit system, in which communication between the patient and the extracorporeal circuit takes place at a vascular access of the Cimino-Brescia fistula type. In this figure, the reference 1 denotes the heart, the reference 2 denotes the pulmonary circuit, the reference 3 denotes the vascular or systemic circuit, and the reference 4 denotes a dialyser which is connected to the systemic circuit 3 by means of a line 5 for withdrawing blood (arterial line) and a line 6 for returning blood (venous line).
As shown in FIG. 1, the blood which is treated during a dialysis session comes from the systemic circuit 3, in which the blood flows with a limited flow rate. For this reason, current dialytic treatments have a limited efficiency, and at the present time there are no corrective means allowing its value to be increased.
Furthermore, the efficiency of dialytic treatments is also reduced by the phenomenon which is known in the medical field by the term xe2x80x9crecirculation in vascular accessxe2x80x9d, the extent of which depends on multiple factors, such as the blood flow rate in the extracorporeal circuit, the position of the needles and the degree of stenosis of the fistula. The recirculation takes place when some of the blood which has been treated in the extracorporeal circuit is recirculated in this circuit via the arterial line 5 immediately after it has been injected into the vascular access 4 via the venous line 6. This phenomenon is illustrated in FIG. 2, where the references 7 and 8 denote the needles for withdrawing and returning the blood, respctively connected to the arterial line 5 and the venous line 6.
The value AR of the recirculation in the vascular access is defined by the following expression:                                           A            R                    ⁢                      xe2x80x83                    ⁢          %                ⁢                  xe2x80x83                =                  xe2x80x83                ⁢                                            Q              R                                      Q              B                                ⁢                      xe2x80x83                    ⁢          100                                    (        1        )            
in which QB is the blood flow rate circulating in the extracorporeal circuit, and QR is the blood flow rate returning into the extracorporeal circuit through the arterial line 5 immediately after the dialytic treatment.
Knowledge of the value AR of the recirculation in the vascular access presents several practical advantages in terms of dialysis treatments: it indicates that the needles 7, 8 should be repositioned when the value AR of the recirculation is too high, and it makes it possible to take action with a view to increasing the precision of the dialytic therapy, to monitor the stenosis of the fistula in the long term, and to increase the mean life of the fistula itself.
There are several known measurement methods for determining the value AR in the vascular access. These can be divided into two broad groups, the first comprising measurement methods which do not involve external action, and the second comprising those which do involve such action.
The first group includes measurement methods which do not involve chemical or physical action on the blood subjected to the dialysis treatment, and which do no more than quantify physiological parameters during the dialysis session.
One example from this group is the method consisting in measuring the urea concentration of three blood samples taken at the same time from the arterial line, the venous line and the patient""s vascular circuit, and in calculating the value AR of the recirculation in the vascular access on the basis of the following equation (equivalent to equation 1):                                           A            R                    ⁢          %                =                                                            C                S                            -                              C                A                                                                    C                S                            -                              C                V                                              ⁢          100                                    (        2        )            
in which CS is the value of the urea concentration in the vascular circulation (systemic concentration), CA is the value of the urea concentration in the arterial line (arterial concentration), and CV is the value of the urea concentration in the venous line (venous concentration).
This method has the drawback of relying on the assumption that, when there is no recirculation in the vascular access, the value of the systemic concentration CS is equal to the value of the arterial concentration CA. However, it has recently been demonstrated that an assumption of this type is not valid under all conditions, and depends on the sampling point. There are therefore differences between these values, which compromise the reliability of the measurement, even when there is no recirculation in the vascular access.
The second group includes measurement methods which provide chemical or physical action on the blood subjected to a dialysis treatment.
An example from this second group is the measurement method which, like the previous one, consists in measuring the blood urea concentration in the arterial line, the venous line and the patient""s vascular circuit. However, unlike in the method mentioned above, when the blood is being sampled from the arterial line in order to determine the value CS of the systemic concentration, the blood flow rate QB circulating in the extracorporeal circuit is set to a minimum in order to limit the recirculation in the fistula as far as possible, and therefore to reduce the differences between the values CS and CA of the systemic concentration and the arterial concentration.
Another example from the second group is the method consisting in imparting a tracer to the patient""s blood in order to obtain chemical or physical dilution of the blood, and in simultaneously using specific sensor means to monitor the progress of the blood in the arterial line or in the venous line, or in both lines. Comparison between the integrals of the signals picked up by the sensors makes it possible to determine the value AR of the recirculation in the vascular access.
In particular, a first known method consists in measuring the temperature of the blood by means of temperature sensors which are arranged along the venous line and along the arterial line, so as to observe the relative temperature profile in response to a quantity of heat (here considered as a tracer) administered to or taken from the blood by means of the dialysis apparatus.
A second known measurement method, based on diluting the blood, is described in U.S. Pat. No. 5,312,550. According to this patent, provision is made for a substance having different physical properties from blood to be injected into the venous line, and for the blood recirculation in the vascular access to be detected by measuring the physical properties of this substance upstream of the point where the substance is injected.
A third known measurement method, based on diluting the blood, is described in U.S. Pat. No. 5,510,717. According to this patent, provision is made for a bolus of a hypertonic solution (tracer) to be injected into the venous line, and for the blood conductivity to be measured using two sensors which are arranged on the venous line and on the arterial line, in order to monitor the relative conductivity variation in response to injection of the bolus.
A fourth known measurement method, based on diluting the blood, consists in injecting a bolus of an isotonic solution (tracer) into the arterial line, upstream of a device for measuring optical absorption which is arranged on this line. The value AR of the recirculation in the vascular access is obtained by comparing the measurement taken by the optical device immediately after injection of the bolus with the measurement which is taken after the bolus has been partially recirculated in the extracorporeal circuit via the arterial access.
The object of the present invention is to produce a method for determining the value of the blood recirculation in an extracorporeal circuit, which is simple and entirely automatic, and which makes it possible to reduce the measurement errors and to use a small number of sensors, which in general already form part of a dialysis apparatus.
To achieve this object, the invention provides a method for determining the blood recirculation in a vascular access of a patient whose blood is subjected to treatment in a blood treatment apparatus, which is connected to the patient by an extracorporeal circuit comprising an arterial line, interconnecting the vascular access with an inlet of the treatment apparatus, and a venous line, interconnecting an outlet of the treatment apparatus with the vascular access, characterized in that it includes the steps of:
inducing a blood concentration in the treatment apparatus;
determining a parameter signifying the blood concentration in the arterial line, for example the blood haemoglobin concentration; and
calculating the blood recirculation on the basis of the measured values of the parameter.
According to one embodiment of the invention, the method comprises the steps of:
setting the blood flow rate in the extracorporeal circuit to a determined value (QB);
determining a first value (HGBA0) of the parameter signifying the blood concentration in the extracorporeal circuit;
inducing a first variation, of determined amplitude (QUF), of the ultrafiltration rate in the treatment apparatus;
determining a second value (HGBA3) of the parameter signifying the blood concentration in the arterial line;
calculating the blood recirculation (AR) in the extracorporeal circuit as a function of the blood flow rate (QB), of the determined amplitude (QUF) of the first variation of the ultrafiltration rate, and of the first and second values (HGBA0, HGBA3) of the parameter signifying the blood concentration in the arterial line.
According to one characteristic of the invention, the step of determining a second value (HGBA3) of the parameter signifying the blood concentration in the arterial line comprises the steps of:
inducing a second variation of the ultrafiltration rate in the treatment apparatus, with the opposite amplitude to the amplitude of the first variation;
determining a third and a fourth value (HGBA1, HGBA2) of the parameter signifying the blood concentration in the arterial line, before the first variation of the ultrafiltration rate and after the second variation of the ultrafiltration rate, respectively;
determining an interpolation line between the third and fourth values (HGBA1, HGBA2) of the parameter signifying the blood concentration;
determining a steady-state value of the parameter signifying the blood concentration in the arterial line after the first variation of the ultrafiltration rate;
correcting the steady-state value of the parameter signifying the blood concentration in the arterial line, on the basis of the interpolation line, in order to obtain the second value (HGBA3) of the parameter signifying the blood concentration in the arterial line.
According to another characteristic of the invention, the treatment apparatus is a dialyser in which a dialysis liquid is circulated, and the circulation of the dialysis liquid is interrupted prior to the step of determining a first value (HGBA0) of the parameter signifying the blood concentration in the extracorporeal circuit.
The invention also relates to a blood treatment system comprising:
a blood treatment apparatus,
an arterial line, intended to interconnect a vascular access of a patient with an inlet of the treatment apparatus,
a venous line intended to interconnect an outlet of the treatment apparatus with the vascular access,
means for inducing a blood concentration in the treatment apparatus;
means which are intended to determine a parameter signifying the blood concentration and are arranged on the arterial line;
characterized in that it includes computation and control means for:
driving the means for inducing a blood concentration,
storing values of the parameter signifying the blood concentration, and
calculating the blood recirculation on the basis of the values which the parameter takes in response to a determined drive program of the means for inducing a blood concentration.